Neither is much good, but the GT 520 is considerably better than the GT210. It's three generations newer and offers 1.5x - 2.0 the performance in terms of fillrate and uses GDDR3 memory instead of either DDR2 or DDR3 which are considerably slower. 500 series cards also support DirectX 11.
If gaming performance is your goal I would strongly suggest that you look to spend a bit more though, these cards offer performance of par with run of the mill graphics cards made 5-6 years ago. They're sufficient for everyday use and movies, but not much else.